I have been thinking about which mutants from Marvel are most realistic and Zeitgeist comes to mind. Long story short, his power is weaponized acidic vomit. He can spew it a few meters away at will.

I am imagining a world where rather than superhero stuff, acidic vomit is something that humans have because it has been positively selected by nature. I imagine it would be useful for self-defense against other animals, and it could also be used in blacksmithing between the stone and iron ages (separating metal from rocks, for example).

However, the genes for acid reflux alone would not make a population more successful. Besides a general tendency to snacking close to bedtime and a taste for spicy and fatty foods and alcohol, what other adaptations could enable for acid vomit as a tool or a weapon?

And if you would be so kind... How much vomit could such a human hold and throw at once, and how far could it be thrown if the human body adapted to maximize these parameters?

$\begingroup$In nature, spitting animals use this ability to capture preys. There is no natural selection reason that would justif this ability in us. So go for good ol' genetic manipulation and come up with the excuse you want.$\endgroup$
– Valerio PastoreJun 20 '18 at 20:13

2 Answers
2

The ability to vomit weapons grade acid involves a highly complex and specific series of mutations in unrelated gene groups. First is the Acid Production, which would require some pretty specific tinkering with the genes related to the development of the stomach's Parietal Cells that are responsible for producing acid. Next it has to be retained without effecting the rest of the digestive process or and also without dissolving the person's abdominal organs themselves. The entire digestive tract would need completely different biochemistry that would most likely radically change the diet, metabolism and nutritional requirements for the now no longer human body. Next the throat, teeth and mouth would likewise have to be somehow altered to not be dissolved by this industrial strength acid. In short, from the lips to the anus your (no longer) human would need to be totally altered just to not be dissolved by it's own acids.

Next you need a method of projecting this acid, which would require a different structure of the abdominal muscles and the abdominal cavity in order to achieve any sort of projectile vomit beyond the meter or so that humans are presently capable of. Assuming all of this restructuring and adaptation hasn't altered the volume of the stomach the average adult has about a 1 liter capacity, however this is the amount held when full, the stomach is not usually filled up to 1 liter capacity with acid. Our digestive system typically ranges from containing .4 to 3 ounces of acid within its 1 liter volume. Not very much actual acid is needed for a human to digest what we do, so assuming your vomitous freaks are all somehow packing a liter of acid they would need to purge their stomach prior to eating. Probably they would pre-digest their food prior to ingesting it in much the same manner as a fly. They would vomit upon whatever they plan to eat, and then drink down the resulting slurry.

Forget Human, Its just Humanoid

The biggest limitation to having a human being able to safely produce weapons grade corrosives and then project them a significant distance to mean anything is the whole human part of the equation. For instance, our mouths are designed for things to go in, not for projecting them out. Our teeth are highly susceptible to decay due to even mild acidic compounds and our esophagus is not really meant for things to go back up it except for on rare occasions when the damage done to the body by vomiting is less than that from failing to purge something toxic or infected. Bulimic individuals often suffer from assorted throat cancers, rapid tooth decay, and damage to their respiratory system. The best way to adapt this particular function is for your creature to simply be humanoid, because the myriad of things that must happen for this to be feasible mean that by the time its all said and done the organism in question is no longer part of the human species.

For a defense mechanism, you're might want to consider mixing something with the acidic vomit somewhere along the line. A more viscous fluid will spread less in the air, will stick to the target better and be harder to wash off - think along the lines of napalm.

The human stomach can hold up to about a gallon when distended.
Bear in mind that's an upper limit after a large meal; you could get about that much volume if they chug water beforehand, but that'll dilute the acid quite a bit.

Because the internet knows all things, I was able to find an article on the distance projectile vomit can travel. At 7.8m for some outlying droplets, that's not too shabby of a defense mechanism even before any modification!

Ultimately, I think stomach acid will hold you back more than anything else; with a pH of 1.5 to 3.5, that concentration of HCl would hurt if it got in a mucous membrane, but it's not going to dissolve skin especially quickly. While not the best source, redditor harpyranchers states that even 12M HCl (which is far stronger than the stuff in your stomach) won't leave immediate burns